The invention relates to a method for preparing natural fibre-based composite materials containing natural binders and powdery proteins.
With this methods, products are made in the form of artefacts such as panels and boards, composed of wood and/or non-wood natural fibre materials and a protein as the sole binder component (added).
Today, manufacture of the panels and boards is generally performed by means of hot pressing of wood or other vegetable fibres in presence of a reactive resin. Typical resins used during the mixing process are urea formaldehyde (UF) resins, phenol formaldehyde (PF) resins and melamine urea formaldehyde (MUF) resins. During the initial blending process aqueous resin is sprayed on the dry wood or vegetable fibres (2-4% moisture) and the whole is blended. The fibres dosed with resin can then be submitted to a further drying step. During this drying process the resin-dosed fibre is pneumatically or mechanically conveyed to the intermediate storage bins. From here, the fibre is transported to the mat forming line and to the pre-press. The thus formed mats are then entering the press and stacked. These mats are then submitted to hot pressing to consolidate the mat to a desirable panel density and thickness, to cure the resins and to heat stabilise the panel so that it will remain at the target thickness and density under normal service conditions. Also continuous presses can be used. The thus obtained panels are then finished by shaving-off the edges by sanding the surfaces.
A disadvantage of this process is that the fibre material must be dried to a sufficient level to allow the blending with the liquid resin; otherwise homogeneous blending is not possible while lump formation occurs at too high moisture content.
An other disadvantage is that when adding the resin, the temperature during the blending and forming step must be controlled, because otherwise, a premature reaction occurs, and this is not desirable.
To reduce or avoid the use of the resin adhesives, the resin adhesives can be partially or fully replaced by renewable resources. The use of animal or vegetable proteins in fibreboard is disclosed in Forest Products Journal (1998, vol.47, n° 2, p. 71-75), wherein the use of soya protein isolates in combination with synthetic resins is discussed. Also the use of soya isolate powder is disclosed. In this method, wood particles are sprayed with 4% phenol formaldehyde resin, whereafter the mixture is further tumbled in a blender while 4% soya isolate powder is slowly added. Also the use of a soya isolate dispersion as the sole adhesive is disclosed.
However, the resultant fibreboard has the disadvantage that it has no moisture resistance.
The use of animal proteins in powder form is further disclosed in Sovjet patent application SU 1 813 640. In the disclosed method, lime milk (1,6-3,2%) and water (35-45%) are added to a fibre material at about 4% moisture content. In a next step, 8-16% albumin or casein powder glue is added, and the whole is mixed until the glue is uniformly distributed into the fibre material. The composition is then heated between 140-170° C. at a pressure of 1,4-2,5 Mpa, this during 0,6-1,0 min/mm.
The use of wheat gluten as an adhesive has been disclosed in Starch (1968, vol2O, no 12 p. 395-399). The gluten that is used is reduced with sodium sulphite or thioglycolic acid.
The disadvantage related to these products is that the wheat gluten adhesive is obtained through a chemical reaction in an aqueous medium.
In Dutch patent application NL 1 003 133, the use of an adhesive based on wheat gluten for preparing fibreboard is disclosed. The wheat gluten glue used is a dispersion at 55-60% d.s., obtained by dispersing gluten into a solution of urea, citric acid and sodium bisulpite. The gluten glue, which is first mixed with a cross-linker (formaldehyde, glutardialdehyde or maleic anhydride), is then sprayed on and mechanically mixed with the wood fibre substrate.
The disadvantage here is that dispersing aids are needed to prepare the gluten glue dispersion.
Also the use of proteins as formaldehyde scavenger is disclosed in Dutch patent application NL 1 003 133, whereby 10% of a standard urea formaldehyde resin is replaced by gluten glue.
The disadvantage is that above 10% replacement of the standard urea formaldehyde resin, there is a clear loss of Internal Bond Strength.
In the European patent application EP 976 790, a process for the manufacture of composite materials is disclosed, in which a vegetable material containing fibre, or a mixture of such vegetable materials, are subjected to at least one thermoplastic processing step. The thermoplastic processing may be carried out in the presence of a bonding agent, for example a chemical bonding agent such as urea-formaldehyde, or a protein (which protein may be contained in the vegetable material or added to it). Prior to the thermoplastic processing step(s), the vegetable material(s) are subjected to at least one preliminary treatment. The product of the thermoplastic processing step(s) may be subjected to an after-treatment. The vegetable material(s) may be mixed with additives prior to and/or during the thermoplastic processing step(s). The composites may be formed in a variety of configurations, including board, sheets and films, and may find use as constructional items.
The disadvantage is that when proteins are used as the sole binder, relative to the total of the fibrous vegetable material and protein, the amount of water may be in the range of 25 to 50% by weight. As a result thereof, the compression-moulded articles must be submitted to an additional time and energy consuming drying step.